A lithium-ion secondary battery includes a cathode, an anode, a porous separator which separates the cathode and the anode. In a process for producing a lithium-ion secondary battery, a separator roll, which is a roll of a separator wound on a cylindrical core, is used. The core is required to have a certain degree of rigidity in order to prevent the core from being deformed by a stress caused by winding the separator on the core.
As a core for winding a film such as a battery separator thereon is known a core having a double cylinder structure in which there are provided a ring-shaped outer peripheral part including an outer peripheral surface around which a film is wound, a ring-shaped inner peripheral part for fitting a take-up roller to an inner peripheral surface of the inner peripheral part, and a plurality of ribs that support the outer peripheral part from the inner peripheral surface, the ribs extending between the outer peripheral part and the inner peripheral part.
Patent Literature 1 discloses a film take-up reel as a core having such a structure. The film take-up reel disclosed in Patent Literature 1 includes an outer wheel (outer peripheral part) having a thickness of 6.15 mm, an inner wheel (inner peripheral part) having a thickness of 5.05 mm, and spoke plates (ribs) each having a thickness of 5 mm, and further includes a reinforcing part protrude through an inner peripheral surface of the outer wheel toward a shaft center. This makes it possible to improve rigidity of the film take-up reel, thus preventing the film take-up reel from being deformed by a stress caused by winding of a film.